A conventional acoustic wave device is described by use of FIG. 7. FIG. 7 is a sectional schematic view of the conventional acoustic wave device. In FIG. 7, conventional acoustic wave device 1 has piezoelectric body 2, interdigital electrode 3 arranged on piezoelectric body 2, and dielectric layer 4 arranged so as to cover interdigital electrode 3.
As related art document information regarding this application, for example, International Publication No. 2007-046236 (Patent Document 1) is known.
It is disclosed that in conventional acoustic wave device 1, a medium where propagation velocity of a transverse wave of SiO2 or the like is relatively low is used as dielectric layer 4, thereby to propagate an acoustic wave with its energy confined on a boundary between piezoelectric body 2 and dielectric layer 4. However, there has been a problem in that, when a film thickness of the dielectric layer is made not smaller than 0.8 times as large as a wavelength of a main wave in an attempt to confine the main wave inside acoustic wave device 1, a spurious radiation by a high-order mode is generated in the vicinity of a frequency of the main wave.